1. Field of the Invention
The present invention relates to an array imaging system, and more particularly to an ultrasonic array imaging system in which ultrasonic echoes reflected from an object to be imaged and received by an array transducer are focused in a digital fashion by applying a bandwidth sampling technique. Such an ultrasonic system can be utilized in medical diagnosis, non-destruction inspection, underwater investigation, etc. Moreover, the present invention can be applied to imaging systems using any type of array transducers including linear, phased, convex, concave and annular arrays. The present invention can be applied not only to ultrasonic imaging systems but also to radar systems using array transducers for the beam forming purpose. The present invention can be applied to any imaging modelity in which the analytic signal can be effectively utilized, including B, C and M mode images, for flow measurement, phase aberration correction, tissue characterization, etc.
2. Description of the Related Art
Generally, in an ultrasonic imaging system, an ultrasonic pulse signal is transmitted toward an object to be imaged, reflected from a surface of an acoustic impedance discontinuity of the object, and received by an array transducer. The received signals are then converted into electric signals which are displayed on a video monitor after various processing. The resulting image conveys some information of the characteristics of the object being examined. In this case, the use of a short pulse increases the signal resolution and the use of an array obtains focusing capability to improve lateral resolution. In the past, various methods have been used to improve the resolution, in particular, the lateral resolution.
FIG. 1 is a schematic block diagram of a conventional ultrasonic imaging system. An ultrasonic pulse signal generated from a pulse generator 11 is supplied to an array transducer 10 via switch 12. The array transducer 10 converts an electric pulse signal into an ultrasonic signal and provides the converted signal to an object 13 to be imaged. Then, the ultrasonic signal is reflected from an acoustic impedance discontinuous surface of the object 13 and the reflected signal is again received by the array transducer 10. At this time, in a case where there are a plurality of acoustic impedance discontinuous surfaces, the ultrasonic signal is in turn reflected from each of discontinuous surfaces and is supplied to a number of transducer elements. The array transducer 10 is composed of a plurality of transducer elements. The ultrasonic signal supplied to the array transducer 10 is converted by transducer elements into an electric signal a with magnitude proportional to an intensity of the ultrasonic signal. The electric signal is amplified to the predetermined magnitude in an amplifier 14 via switch 12, processed to a video signal in a signal processor 15, and transmitted to a cathod ray tube (CRT) 16.
In such an ultrasonic imaging system, the array transducer of a probe is composed of a plurality of transducer elements in the array form. This array form improves the resolution of a picture to be displayed with focusing an ultrasonic signal. Ultrasonic signals, which are reflected from the object and supplied to the array transducer, reach the transducer elements at different times according to the positions of the respective transducer elements. That is, the farther their positions are from the middle of the array transducer 10, the amount of time necessary to reach the transducer elements increases. Even if signal focusing can be performed at a transmission focusing step of transmitting the ultrasonic signal from the array transducer 10 to the object, it is preferred that signal focusing occur at a receiving focusing step, capable of dynamic focusing, rather than the former step. In receiving focusing, differences between reach times should be respectively compensated after delayed, they are so as to focus electric signals output from the transducer elements.
FIG. 2 illustrates one embodiment of a conventional receiving focusing device of an ultrasonic signal. The device of FIG. 2 utilizes analog delayers. An array transducer 20 is composed, of n transducer elements in numbers, in which receiving signals converted into electric signals by each of transducer elements are supplied to n delayers DLY1.about.DLYn, respectively. Among ultrasonic signals reflected from an object and supplied to the array transducer 20, each of delayers DLY1.about.DLYn allows the longest delay time to an input signal of the middle transducer element with the shortest reach time and permits the shortest delay time to input signals of first and n transducer elements with the longest reach time. Therefore, the delayers DLY1.about.DLYn simultaneously output the delayed signals therein to an adder 22. The signals output from the respective delayers are added in the adder 22 and outputted as a focusing signal. The conventional device of FIG. 2 connects the delayers, having a predetermined delay time value, to output terminals of each of transducer elements and delays the output signal from the transducer elements. Thus, each of delayers DLY1.about.DLYn compensates for differences of reach time to the transducer elements of the ultrasonic signals reflected from the object. An output signal from the adder 22 is supplied to an envelope detector 23. However, in order to reduce errors during compensating of delay time, the conventional device of FIG. 2 employs a plurality of taps in the delayers. As such, such a device requires complicated hardware. Also, in the case of dynamic focusing, the delay time of each delayer should be changed. Accordingly, the more the number of focal points increases, the more complicated the hardware becomes. Moreover, impedance mismatching causes a reduction of the range dynamic focusing and a decrease of resolution therefrom.
FIG. 3 illustrates another embodiment of a conventional receiving focusing device of an ultrasonic signal. The device of FIG. 3 utilizing what is called pipelined sampled delay focusing (PSDF). In the device of FIG. 3, an array transducer 30 is composed of n transducer elements in numbers. Received ultrasonic signals are converted into electric signals by each of transducer elements. The electric signals output from transducer elements of the array transducer 30 are respectively supplied to n analog to digital converters A/D1.about.A/Dn of an analog to digital converting unit 31. A clock generator 32 generates sampling clocks of a frequency "fs". The A/D converting unit 31 converts each of electric signals which are input from the transducer elements according to the sampling clock into digital signals, and supplies the converted signal to memories FIFO1.about.FIFOn, respectively. The sampling clock of the clock generator 32 is not a uniform clock, but a variable sampling clock. That is, in the case of the dynamic focusing, the reach time of the ultrasonic signal is different from each other according to positions of each focal point. Output terminals of the A/D converting unit 31 are connected to a memory unit 33 composed of first-in-first-out memories FIFO1.about.FIFOn. Output data of the A/D converting unit 31 is arranged within the memory unit 33, to be output in input order. Therefore, the ultrasonic signals, which are reflected from a particular focal point and supplied to transducer elements, can be simultaneously output from the memory unit 33. After initial data corresponding to a specific focal point is supplied to the memory unit 33 and a maximum delay time goes by, data is simultaneously output from the memory unit 33. The output data is added in an adder 34, and then dynamic focusing is performed. The focused ultrasonic signal is converted into an analog signal by digital-to-analog converter 35 and the converted signal is supplied to an envelope detector 36. The envelope detector 36 detects an envelope from the input signal, and an analog-to-digital converter 37 converts the detected envelope into a digital signal.
Using such a technique, it is advantageous that the conventional device can obtain the best resolution by focusing an ultrasonic signal on all focal points requiring the dynamic focusing. However, it is disadvantageous that the level of a sampling frequency becomes high because the A/D converting unit 31 samples a radio frequency signal. For instance, in the case of an array transducer of 3.5 MHz, a sampling frequency is required beyond 28 MHz. Therefore, such a of system suffers from the problem of noise due to high sampling frequencies of A/D conversion. Furthermore, a problem with relatively expensive costs is encountered because both memories and peripheral circuits with a high speed device should be employed. In addition, since a final signal required in an ultrasonic video device is an envelope of a focused RF signal, not the focused RF signal, the detection of the envelope should be executed. However, it is difficult to detect the envelope from digital data which is sampled to a high frequency beyond 28 MHz. So, as shown in FIG. 3 the envelope detection should be executed after converting the focused digital RF data into analog data again. Accordingly, new noise may occur in a process of D/A conversion.